Negative electrodes for lithium cells and batteries

ABSTRACT

A negative electrode is disclosed for a non-aqueous electrochemical cell. The electrode has an intermetallic compound as its basic structural unit with the formula M 2 M′ in which M and M′ are selected from two or more metal elements including Si, and the M 2 M′ structure is a Cu 2 Sb-type structure. Preferably M is Cu, Mn and/or Li, and M′ is Sb. Also disclosed is a non-aqueous electrochemical cell having a negative electrode of the type described, an electrolyte and a positive electrode. A plurality of cells may be arranged to form a battery.

RELATED APPLICATIONS

[0001] This application, pursuant to 37 C.F.R. §1.78(c), claims prioritybased on provisional application serial no. 60/267,512 filed Feb. 8,2001.

[0002] The United States Government has rights in this inventionpursuant to Contract No. W-31-109-ENG-38 between the U.S. Department ofEnergy (DOE) and The University of Chicago representing Argonne NationalLaboratory.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

[0003] This invention relates to new intermetallic negative electrodes(anodes) for lithium batteries. According to the invention, theintermetallic electrodes are based on a formulation that for a binarysystem of two different metal elements, M and M′, including Si, can berepresented M₂M′ with a Cu₂Sb-type structure. The invention extends toinclude ternary, quaternary and higher order intermetallic electrodes inwhich the M and M′ atoms can be of more than one atom type. Theintermetallic compounds of the present invention need not be orderedsystems; that is, they may have fully disordered structures in which theM or M′ atoms are arranged in a random manner, or they may havepartially disordered structures in which the M or M′ atoms in thecrystal lattice are arranged in a non-random manner. The M atoms of thisinvention are preferentially Cu, Li and/or Mn atoms, and the M′ atomsare preferentially Sb atoms.

SUMMARY OF THE INVENTION

[0004] Rechargeable lithium cells (and batteries), commonly referred toas lithium-ion cells (and batteries) have found widespread applicationfor powering devices such as cellular phones, laptop and hand-heldcomputers and camcorders; they are also of interest for largerapplications such as stand-by energy storage, electric andhybrid-electric vehicles. The most common lithium-ion cell has theconfiguration Li_(x)C/electrolyte/Li_(1−x)CoO₂. During charge anddischarge, lithium ions are shuttled electrochemically between two hostelectrode structures that consist of a carbonaceous (typicallygraphitic) Li_(x)C anode and a layered Li_(1−x)CoO₂ cathode. These cellsare inherently unsafe, particularly if heated in a charged state or ifthey are overcharged without protective electronic circuitry. Lithiatedgraphite electrodes operate at a potential very close to that ofmetallic lithium and are extremely reactive. There is, therefore, a needto find alternative electrodes to graphite.

[0005] According to this invention, there is described a newintermetallic structure type that can be effectively used as a negativeelectrode (anode) for a non-aqueous lithium electrochemical cell and/orbattery. These new intermetallic electrodes have as their basicstructural unit the formula M₂M′ in which M and M′ are selected from twoor more metal elements including Si, and have a Cu₂Sb-type structure. Inthis structure type, the M′ atoms of the M₂M′ structure have aconfiguration that is close to an ideal face-centered-cubic array ofatoms, and provide a host framework for the M atoms in the parentstructure and for Li atoms during the electrochemical reaction. The Matoms of this invention are preferably Cu, Mn and/or Li atoms, and theM′ atoms are preferably Sb atoms.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The invention consists of certain novel features and acombination of parts hereinafter fully described, illustrated in theaccompanying drawings, and particularly pointed out in the appendedclaims, it being understood that various changes in the details may bemade without departing from the spirit, or sacrificing any of theadvantages of the present invention.

[0007]FIG. 1 depicts a schematic illustration of the M₂M′ structure ofthe invention,

[0008]FIG. 2 depicts the voltage profile for the first cycle of aLi/Cu₂Sb cell;

[0009]FIG. 3 depicts the X-ray diffraction patterns of the Cu₂Sbelectrode, collected in situ, in the Li/Cu₂Sb cell of FIG. 2;

[0010]FIG. 4 depicts a plot of capacity vs. cycle number for the first25 cycles of a Li/Cu₂Sb cell;

[0011]FIG. 5 shows the X-ray diffraction patterns of a Mn₂Sb electrode;

[0012]FIG. 6 shows the voltage profile of the first discharge/charge ofa Li/Mn₂Sb cell;

[0013]FIG. 7 depicts a plot of capacity vs. cycle number for the first23 cycles of the Li/Mn₂Sb cell of FIG. 6;

[0014] FIGS. 8(a)-(d) depict schematic illustrations of the structuresformed during the electrochemical transformation of Cu₂Sb to Li₃Sb withFIG. 8a being in the 100 projection and FIG. 8c being in the 110projection; and

[0015]FIG. 9 depicts a schematic representation of an electrochemicalcell; and

[0016]FIG. 10 depicts a schematic representation of a battery consistingof a plurality of cells connected electrically in series and inparallel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] A renewed interest in intermetallic negative electrodes asalternatives to graphite for lithium-ion batteries has developed overrecent years because of the inherent safety hazards of these cells,particularly when subject to abuse or overcharge conditions. Binarylithium-metal systems such as Li_(x)Al, Li_(x)Si and Li_(x)Sn have notyet found practical application because the expansion and contractionthat occurs during lithiation and delithiation is so large that thestructural integrity and cycle life of the electrode is impaired asdescribed by Huggins in: Handbook of Battery Materials (Editor: J. O.Besenhard), Wiley-VCH, Wienheim, Germany, Part III.1, page 359 (1999),the disclosure of which is incorporated by reference. One of the mostcommon approaches to alleviate this problem is to embed anelectrochemically active intermetallic electrode in a composite matrix.(By intermetallic electrode, it is meant that the electrode includes twoor more metal elements including silicon.) Perhaps the best knowncomposite system is derived from tin oxide, in which domains oflithiated tin, Li_(x)Sn, are created within an electronically insulatingLi₂O matrix during the electrochemical reaction with lithium, a conceptthat was reported by Y. Idota et al in Science, Volume 276, page 570(1997), the disclosure of which is incorporated by reference. A majorproblem with this system is that the electrode is subject to anunacceptably large irreversible capacity loss, which is attributed tothe inability of the lithium trapped within the Li₂O matrix to partakein the electrochemical reaction. There have been extensions to thisconcept, for example, to use two electronically conducting intermetalliccomponents, one “active”, such as FeSn₂, and the other “inactive”, suchas Fe₃SnC as described by O. Mao et al in Electrochemical and SolidState Letters, Volume 2, page 3 (1999), the disclosure of which isincorporated by reference. It has also been recognized that in order toobtain high capacities and high rates, the particle size of thesecomposite electrodes should be as small as possible, preferably withnanoscale dimensions. However, a disadvantage of this approach is thatextremely small electrochemically active particles will be more prone toreaction with the organic-based electrolytes of lithium cells thanlarger sized particles. In another development, intermetallic compoundshave been identified that operate by lithium insertion/metal extrusionreactions, the most notable being Cu₆Sn₅ with a nickel-arsenide-typestructure that transforms to Li_(4.4)Sn via a Li₂CuSn lithiatedzinc-blende-type intermediate structure as reported by Kepler et al inElectrochemical Solid State Letters, Volume 2, page 307 (1999), thedisclosure of which is incorporated by reference and InSb with azinc-blende-type structure that transforms on complete lithiation toLi₃Sb, as reported by Vaughey et al in Electrochemistry and Solid StateLetters, Volume 3, page 13 (2000), the disclosure of which isincorporated by reference. The latter two examples described above havedrawn attention to the existence of a new class of intermetalliccompounds in which a strong structural relationship exists between aparent compound and its lithiated products. These types of compoundshold promise for improving the kinetics and reversibility ofintermetallic anodes and for overcoming the safety limitations oflithiated graphite electrodes of state-of-the-art lithium-ion cells andbatteries. However, further improvements in structural design are stillrequired before such compounds can compete with graphite electrodes inlithium-ion cells and batteries.

[0018] According to this invention, there is described a newintermetallic structure type that can be effectively used as a negativeelectrode (anode) for a non-aqueous lithium electrochemical cell and/orbattery. These new intermetallic electrodes have as their basicstructural unit the formula M₂M′ in which M and M′ are selected from twoor more metals including Si, and the M₂M′ structure is a Cu₂Sb-typestructure, as illustrated in FIG. 1. In this structure type, the M′atoms of the M₂M′ structure may have a configuration that is slightlydistorted from an ideal face-centered-cubic array of atoms, and providea host framework for the M atoms in the parent structure and for Liatoms during the electrochemical reaction. Lithium insertion intobinary, ternary or higher order intermetallic compounds is oftenaccompanied by the extrusion of metal atoms from the parent structure,but this is not necessarily always the case. When the M atoms areextruded from an M₂M′ electrode structure of this invention, then theelectrode may be considered as a composite electrode that consistsessentially of a matrix containing intermetallic Li_(x)M_(2−y)M′particles having a host M′ lattice containing x Li atoms and theremaining (2−y) M atoms, in intimate contact with the y M atoms that areextruded from the structure. The M atoms of this invention arepreferably Cu, Mn and/or Li atoms, and the M′ atoms are preferably Sbatoms, but may include Group Va elements excluding nitrogen.

[0019] In one embodiment of the invention, the parent intermetallic M₂M′electrode can be a binary system of two different atom types, M and M′,or it can be a ternary, a quaternary or a higher order intermetallicsystem in which the M and M′ atoms can be of one atom type, or more thanone atom type. The intermetallic compounds of the present invention neednot be ordered systems; that is, they may have fully disorderedstructures in which the M or M′ atoms are arranged in a random manner,or they may have partially disordered structures in which the M or M′atoms in the crystal lattice are arranged in a non-random manner.Furthermore, the M₂M′ compound of the invention need not be preciselystoichiometric, such that the M:M′ ratio in the parent structure canfall within the range 2.33:1 to 1.67:1, and preferably within the range2.1:1 to 1.9:1.

[0020] According to a second embodiment of the invention, the metalatoms of the intermetallic electrodes can be metals that are eitheractive toward lithium, such as Sb, or inactive toward lithium, such asCu or Mn, or the M₂M′ compound may be selected in which both M and M′are electrochemically active toward lithium. Preferably, for example, ina binary system M₂M′, the M atoms are substantially inactive towardlithium, such as Cu, and the M′ atoms are substantially active towardlithium, such as Sb, which together with Li forms Li₃Sb. The M atoms arealso preferably inactive toward the electrolyte.

[0021] One of the problems of intermetallic electrodes is that on theinitial reaction with lithium, they tend to show an irreversiblecapacity loss, i.e., the capacity that is delivered on the initialinsertion of lithium into the intermetallic host structure cannot berecovered during the subsequent lithium extraction reaction. Althoughnot yet fully understood, this irreversible capacity loss is believed tobe at least partly a result of the inability for all the extruded metalto be reincorporated into the structure, and partly by a passivationlayer that is created on the surface of the lithiated intermetallicelectrode, which renders some of the lithium inactive for furtherparticipation in the electrochemical reaction. Therefore, in a thirdembodiment of the invention, these effects may be countered by addingsurplus M metal in finely divided form to the initial M₂M′ electrode ofthe invention, typically to the extent of 50 atom percent or less,preferably 20 atom percent or less. Alternatively, these effects may becountered by including some additional lithium in the parent M₂M′electrode structure either as a separate component M₂M′, or by replacingsome M or M′ in the M₂M′ parent structure, for example, asM_(2−δ)Li_(δ)M′, in which δ is preferably less than 20 atom percent ofthe M atom content.

[0022] According to a further aspect of the invention, theelectrochemical reaction may take place by lithium insertion into, andmetal extrusion from, the face-centered-cubic M′ metal array, such thatthe face-centered-cubic array is maintained during the electrochemicalreaction. The reaction may also be one that proceeds to form Li_(x)MM′structures in which MM′ may be a zinc-blende-type framework. Furtherextrusion of the remaining M atoms from the Li_(x)MM′ structures canalso be possible, while maintaining the face-centered-cubic M′ metalarray.

[0023] Compounds with the Cu₂Sb-type structure, in which the M′ atom isa non-metal, such as Fe₂As, Cr₂As, Cu_((4−x))Te₂, LiFeAs, LiFeP, andLiCoAs are also known to exist as reported by Wyckoff in CrystalStructures, second edition, Wiley, Volume 1, page 361 (1965) and by Juzaet al in Zeitschrift für Anorganische und Allegemeine Chemie, Volume361, page 58 (1968) the disclosures of which are incorporated byreference. It is of particular significance that in the Li—Fe—As system,both Fe₂As and LiFeAs have a Cu₂Sb-type structure, and that a solidsolution between Fe₂As and LiFeAs exists. In this instance, it maytherefore be expected that when lithium is reacted electrochemicallywith Fe₂As, lithium will be inserted into, and iron extruded from, theAs array and that the inserted Li atoms in the LiFeAs product willreside in the same crystallographic positions that were previouslyoccupied by the extruded Fe atoms. Therefore, in a fourth embodiment ofthe invention, the M₂M′ electrode structure may be one in which M isselected from one or more metal elements and M′ is selected from one ormore metal or non-metal elements, preferably such that M′ is selectedfrom the Group Va elements, other than nitrogen, for example, P and/orAs.

[0024] The principles of this invention are described with particularreference to the two isostructural compounds, Cu₂Sb and Mn₂Sb. These twoexamples of intermetallic anodes describe the principles of theinvention as contemplated by the inventors, but they are not to beconstrued as limiting examples.

EXAMPLE 1

[0025] Cu₂Sb was synthesized by ball milling stoichiometric amounts ofmetallic copper and antimony with 5 at % graphite as a solid lubricantfor 17 h in air using a SPEX/CertiPrep high-energy ball mill. Theresultant powder was sieved through a mesh screen to isolate theelectrode particles with size less than 75 μm.

EXAMPLE 2

[0026] Laminates of Cu₂Sb electrodes were fabricated by mixing 84 wt %Cu₂Sb as made in Example 1 with 8 wt % carbon (acetylene black) and 8 wt% polyvinylidine difluoride (PVDF). The electrode slurry was extrudedonto copper-foil and vacuum-dried at 120° C. for at least 5 h prior touse. Two-electrode cells were assembled in an argon filled glove-box (O₂and H₂O<5 ppm) using Cu₂Sb as the working electrode and lithium as thecounter electrode, separated by a glass fibre membrane soaked inelectrolyte. A 1M LiPF₆ EC/DEC (ethylene carbonate/diethyl carbonate)(2:1) solution was used as electrolyte. Li/Cu₂Sb cells were housed in analuminum foil container.

[0027] The voltage profile of the first cycle of a Li/Cu₂Sb cell, whichwas used for the in-situ X-ray diffraction data collection of the Cu₂Sbelectrode, is shown in FIG. 2. In situ X-ray diffraction data of theCu₂Sb electrode were collected during the initial discharge/charge cyclein transmission mode using a STOE & CIE GmbH STADI powder diffractometerfitted with a position-sensitive detector (CuKα₁ radiation). Eachmeasurement was recorded between 20° and 55° in 2θ. The Li/Cu₂Sb cellwas discharged and charged in potentiostatic mode on a MacPilell™instrument with steps of 10 mV. The cell was allowed to equilibratebefore each diffraction pattern was recorded. The powder X-raydiffraction patterns of the parent Cu₂Sb electrode and the electrode atvarious states of discharge and charge are shown in FIG. 3. Unit cellparameters of the Cu₂Sb electrode and its lithiated products werecalculated by a least-squares refinement of the peak positions in theX-ray diffraction patterns of FIG. 2 (CuKα radiation). Cell cycling wasperformed on a second Li/Cu₂Sb cell on a Digatron BTS-600 battery testerin galvanostatic mode with a current density of 0.2 mA/cm². Cells werecycled between 1.2 and 0.0 V. The capacity versus cycle number plot forthe first 25 cycles of a second Li/Cu₂Sb cell is shown in FIG. 4.

EXAMPLE 3

[0028] Mn₂Sb was synthesized by ball milling stoichiometric amounts ofmetallic manganese and antimony with 5 at % graphite as a solidlubricant for 17 h in air using a SPEX/CertiPrep high-energy ball mill.The resultant powder was sieved through a mesh screen to isolate theparticles with size less than 75 μm. The powder X-ray diffractionpattern of the Mn₂Sb product is shown in FIG. 5 (CuKα radiation).

EXAMPLE 4

[0029] Laminates of Mn₂Sb electrodes were fabricated by the sameprocedure described in Example 2 for Cu₂Sb. For the electrochemicalevaluation, Li/Mn₂Sb cells of size 2016 (i.e., 20 mm diameter; 1.6 mmhigh) were used. Cells were cycled in galvanostatic mode between 1.5 and0 V using a current density of 0.4 mA on an automated Maccor celltesting system. The voltage profile for the first cycle of a typicalLi/Mn₂Sb cell is shown in FIG. 6. The capacity versus cycle number plotfor the first 25 cycles of this cell, cycled between 1.5 and 0 V isshown in FIG. 7.

[0030] The X-ray diffraction data of a Li/Cu₂Sb cell were obtained insitu at intermittent intervals during the initial discharge and charge.The voltage profile of this cell is shown in FIG. 2, in which points Ato J indicate the voltages at which the X-ray data were collected; thecorresponding X-ray diffraction patterns are provided in FIG. 3. Peaksin the diffraction patterns that originate from the cell hardware, suchas the copper current collector and the aluminum foil container, aremarked in the figure. The voltage profile is divided into sectionsI-VII, which indicate the various stages of the reaction. Beforecycling, the open-circuit voltage of the Li/Cu₂Sb cell was 3.0 V and at1.05 V. The X-ray diffraction peaks from the initial Cu₂Sb electrode arevisible at approximately 26, 29, 31.5, and 35° 2θ; additional peaks fromCu₂Sb between 44° and 45° 2θare overlapped by peaks from the cellhardware. The Cu₂Sb peaks remain unchanged during discharge down to 1.05V (Point A). Section I, between 3.0 and 0.9 V in the voltage profile,accounts for a capacity of approximately 90 mAh/g which is lost on thesubsequent cycle. This loss in capacity is attributed to the reaction oflithium with an electrochemically active surface of the Cu₂Sb particles,possibly an oxide that is formed during the synthesis of Cu₂Sb byball-milling. At 0.82 V (point B), the end of section II, the Cu₂Sbpeaks are replaced by new peaks at approximately 25, 41 and 48° 2θ;these peaks are consistent with a cubic structure having a CuSbzinc-blende-type framework. On further discharge to point D at 0.70 V(Section III), via point C at 0.74 V, the diffraction peaks of thelithiated electrode shift very gradually towards lower 2θ. Sections IIand III are, therefore, attributed to an electrode systemLi_(x)Cu_(2−y)Sb (0<x≦2, 0<y≦1) with end member Li₂CuSb at x=2, y=1. At0.70 V, a voltage plateau (Section IV) signifies the onset of atwo-phase reaction consistent with the formation of a Li_(2+z)Cu_(1−z)Sbstructure (0<z<1), closely related to Li₃Sb, as evident by the peaksthat appear at approximately 23, 27, 39 and 46° 2θ at the expense of theLi₂CuSb peaks, that become more pronounced on further discharge to 0.61V and 0.41 V in FIG. 3. At 0.61 V (point E) and at lower voltages (pointF) only the Li_(2+y)Cu_(1−y)Sb phase is present (this is also true whencells reach 0 V, not shown in FIG. 3. The capacity at the lowerpotentials is attributed to the continued extrusion of Cu from thestructure, yielding the final discharge products Li₃Sb and Cu at y=1 andLi_(2+y)Cu_(1−y)Sb . The extruded copper could not be detected in theX-ray diffraction patterns because they coincide with those of thecopper current collector. High resolution images of the lithiated Cu₂Sbelectrodes obtained on a transmission electron microscope have shownthat the extruded Cu remains closely connected to the antimony matrix asvery small needle-like crystals, a few hundred nanometers in length;this feature is believed, to be amongst the reasons for the excellentreversibility of cells when cycled between 1.2 and 0 V, as shown in FIG.4.

[0031] The crystallographic parameters for Cu₂Sb (pristine electrode),“Li₂CuSb”calculated at 0.7 V (FIG. 3) and “Li₃Sb” calculated at 0.41 V(FIG. 3) are provided in Table 1. “Li₂CuSb” and “Li₃Sb” are put inparentheses because the precise compositions of the electrodes at whichthe lattice parameters were calculated are not known. Nevertheless, thecalculated values in Table 1 are in good agreement with those reportedpreviously in the literature for Cu₂Sb, a=3.992 Å, c=6.091 Å; Li₂CuSb,a=6.268 Å; and Li₃Sb, a=6.573 Å. Using the crystallographic parametersin Table 1, the volume expansion for the transition of Cu₂Sb to“Li₂CuSb” is 25.2%. During the cubic-cubic transition of “Li₂CuSb” to“Li₃Sb”, the unit cell parameter, a, increases by 4.3%, corresponding toa unit cell volume expansion 13.3%. The total volume expansion of the Sbarray for the complete Cu₂Sb to “Li₃Sb” transformation is 42%. TABLE 1Crystallographic parameters for Cu₂Sb, Li₂CuSb and Li₃Sb. Space a b cUnit Cell Compound Group (Å) (Å) (Å) Z Volume (Å³) Cu₂Sb P4/nmm 4.03(1)4.03(1) 6.10(12) 2 99 “Li₂CuSb” F-43m 6.28(2) 6.28(2) 6.28(2)  4 248“Li₃Sb” Fm3m 6.55(1) 6.55(1) 6.55(1)  4 281

[0032] The electrochemical processes that occur in sections II, III andIV are reversible; during charge, in region V, the in-situ X-raydiffraction data show that the Li₂CuSb phase is reformed by 0.93 V(point G, FIG. 3). Further delithiation to 1.01 V (region VI, to pointI), shows a gradual shift of the Li₂CuSb peaks towards higher 20consistent with the formation of Li_(2−x)Cu_(1+y)Sb compositions (0<x<2,0≦y<1). The X-ray diffraction data obtained for the final process to1.05 V (region VII, to point J) are consistent with the regeneration ofa Cu₂Sb-type structure.

[0033] A capacity vs. cycle plot for the first 25 cycles of a Li/Cu₂Sbcell is provided in FIG. 4. The first cycle has a large irreversiblecapacity, consistent with the data from the in-situ cell (FIG. 1).Thereafter, Cu₂Sb shows excellent cycling stability with a steadycapacity of approximately 290 mAh/g. On the initial conditioning cycle,there is a 36% capacity loss, whereas from cycle 2 to cycle 25 thecapacity loss is 0.33% per cycle, and from cycle 4 to 25 it is 0.14% percycle. The theoretical gravimetric capacity for the complete reaction

3Li+Cu₂Sb→Li₃Sb+2Cu  (2)

[0034] is 323 mAh/g. Therefore, the delivered rechargeable capacity of290 mAh/g reflects a very high utilization of the electrode (90%). Thecrystallographic density of Cu₂Sb is high (8.51 g/ml), whereas thedensity of the Li₃Sb/Cu composite electrode at the end of discharge isconsiderably lower (4.70 g/ml). The theoretical volumetric capacity forCu₂Sb, based on the average density of the electrode (6.60 g/ml) is 2132mAh/ml, which is significantly higher than the theoretical volumetriccapacity of graphite (818 mAh/ml), which is the preferred negativeelectrode for current Li-ion cells.

[0035] With the available in situ X-ray diffraction and crystallographicdata, the following reaction sequence for the lithiation of Cu₂Sb, whichis represented schematically in FIGS. 8(a)-(d) is proposed. Theprojections of the structures in FIGS. 8(a)-(d) and compositions of theproducts have been selected to simplify the illustration of the overallreaction model. In this respect, it is acknowledged that, because of theeasy exchange between Li and Cu atoms in Li_(x) Cu_(2−y)Sb structures(x≦3, y≦2), deviations from the ideal stoichiometric compositions, asillustrated in FIGS. 8(a)-(d), can be expected.

[0036] The first stage of the reaction of lithium with Cu₂Sb (SectionsII and III in FIG. 2) during which there is a transformation to a cubicCuSb zinc-blende framework can be represented

xLi+Cu₂Sb→Li_(x)Cu_(2−y)Sb+y Cu  (3)

[0037] for 0<x≦2, 0<y<1, yielding Li₂CuSb at x=2 and y=1. The Cu₂Sbstructure is tetragonal (space group P4/nmm) with Cu located on the2a(000) and 2c(0½0.27) sites, and Sb on the 2c(0½0.70) site. Therefore,in the [100] projection, the Cu₂Sb structure can be visualized as beingcomprised of Cu layers with alternating layers of Cu and Sb (FIG. 8a);the Cu and Sb atoms are arranged in discrete columns down the a axis ofthe unit cell. The following processes occur during the transformationof Cu₂Sb to the CuSb zinc-blende framework, as shown in FIG. 8(a-c): 1)the Sb atoms, that are arranged in a slightly distorted face-centeredarray in the parent tetragonal Cu₂Sb structure undergo smalldisplacements to create an ideal face-centered-cubic array in the CuSbzinc-blende framework; 2) one-half of the copper atoms are extruded fromthe Cu₂Sb structure; 3) one-quarter of the copper atoms remainessentially in their original positions, and one-quarter are displacedin the a and c directions of the Cu₂Sb unit cell to create the puckeredhexagons of the CuSb zinc-blende framework as shown in a [110]projection in FIG. 8c; 4) the Sb and Cu displacements cause acompression of the c-axis and an expansion of the a-axis in Cu₂Sb toyield the cubic CuSb zinc-blende framework; 5) for every extruded Cuatom, two Li atoms are accommodated in the interstitial space of theCuSb framework to yield Li₂CuSb (space group F-43m), which isisostructural with Li₂CuSn, derived by lithiation of Cu₆Sn₅ with aNiAs-type structure as reported by Kepler et al in Electrochemical SolidState Letters, Volume 2, page 307 (1999). FIG. 8(a-c) demonstrate thatthere is a strong structural relationship between the parent Cu₂Sb andthe CuSb zinc-blende framework as there is between Cu₆Sn₅ and the CuSnzinc-blende framework. In this respect, it should be noted that in thehexagons of the CuSb framework, all the Sb atoms and one-half of the Cuatoms are close to their original positions in the parent Cu₂Sbstructure.

[0038] The electrochemical profile and the lattice parameter of theLi_(x)Cu_(2−y)Sb electrode shown in Sections II and III, in FIG. 2 and3, respectively, show more variation than would be suggested by a simpletwo-phase Cu₂Sb-Li₂CuSb system. A least squares refinement of thelattice parameter at point B at 0.82 V in FIG. 3, yielded a value ofa=6.26(1) Å, which is 0.3% less than that calculated for “Li₂CuSb” atpoint D at 0.7 V (6.28(2) Å, Table 1).

[0039] The second stage of the reaction of lithium with Cu₂Sb (SectionIV, FIG. 2) occurs first on a voltage plateau at approximately 0.7 V,and then with sloping voltage to 0 V; this stage is attributed to thesubstitution of copper by lithium in the face-centered-cubic Sb array asshown in a [110] projection in FIG. 8(c). The reaction for the completesubstitution of Cu by Li is:

Li+Li₂CuSb→Li₃Sb+Cu  (4)

[0040] However, the electrochemical profile and X-ray diffraction datain FIGS. 2 and 3 suggest that reaction (4) occurs by a similarinhomogeneous process of lithium insertion and copper extrusiondescribed for reaction (3) above. Reaction (4) can, therefore, bewritten more generally as:

y Li+Li₂CuSb→Li_(2+y)Cu_(1−y)Sb+y Cu  (5)

[0041] for 0<y≦1). The precise composition of the reaction product forthe two-phase process at 0.7 V, (i.e., the value of y) is unknown; thelattice parameter determined from the in situ X-ray diffraction data(e.g., a=6.55(1) Å at 0.41 V, Table 1) suggest a composition close toLi₃Sb (a=6.573 Å). Between 0.7 and 0 V, it appears that the residual Cuis extruded continuously from the Li_(2+x)Cu_(1−x)Sb electrode structureto yield Li₃Sb (space group Fm3m, FIG. 8d); a small contribution to thecapacity from the added acetylene black current collector over thisvoltage range cannot be discounted.

[0042] Therefore, Li/Cu₂Sb cells operate by a mechanism involvinglithium insertion/copper extrusion reactions with the Cu₂Sb electrode.The overall reaction can be described by the general process:

x Li+Cu₂Sb⇄Li_(x)Cu_(2−y)Sb+y Cu  (6)

[0043] for 0<x≦3 and 0<y≦2. The excellent cycling stability of the cellsin which the Cu₂Sb electrode provides a rechargeable capacity of 290mAh/g (alternatively, 1914 mAh/ml based on an average electrode densityof 6.6 g/ml) can be attributed to two main factors. First, there arestrong structural relationships between the lithium-copper-antimonyphases, Cu₂Sb, Li₂CuSb and Li₃Sb formed during the electrochemicaldischarge and charge reactions, the compositions of which can varybecause of the possible exchange of Li for Cu in the Sb array. Ofparticular significance is the retention of an essentially invarianthost Sb array for Li and Cu throughout the reaction process. Second, theCu₂Sb electrode from which finely divided copper is extruded providesgood electronic conductivity at all states of charge, thereby providingan electrode with low cell impedance and fast reaction kinetics.

[0044] The X-ray diffraction data of a Mn₂Sb electrode is shown in FIG.5; the voltage profile of the first cycle of a lithium cell containingthis electrode is shown in FIG. 6. Although in situ X-ray diffractiondata collected to date have not revealed the precise nature of theintermediate discharge products that are formed above the plateau at 0.4V in FIG. 6, the X-ray data have shown that Li₃Sb is formed on a flatvoltage plateau at approximately 0.4 V and between 0.4 V and 0 V.Capacity can be recovered during the charge process, but withconsiderably greater hysteresis than observed for Cu₂Sb electrodes. Thishysteresis can be attributed to slower diffusion of Mn atoms within theSb array of the electrode structure compared to Cu atoms. In situ X-raydiffraction data collected during the charge process to 1.5 V in FIG. 6have indicated that LiMnSb- and MnSb-type structures may form asintermediate products during the charge process and that theregeneration of Mn₂Sb is more difficult to achieve than is the case forCu₂Sb electrodes. Nevertheless, after one conditioning cycle Mn₂Sbelectrodes can deliver a rechargeable capacity in excess of 240 mAh/gfor 23 cycles as shown in FIG. 7. These electrochemical data emphasizethe superior electrochemical performance of Cu₂Sb electrodes over Mn₂Sbelectrodes, thereby making Cu₂Sb a preferred electrode material.

[0045] This invention, therefore, relates to an intermetallic negativeelectrode (anode) for a non-aqueous electrochemical lithium cell asshown schematically in FIG. 9, the cell represented by the numeral 10having a negative electrode 12 separated from a positive electrode 16 byan electrolyte 14, all contained in an insulating housing 18 withsuitable terminals (not shown) being provided in electronic contact withthe negative electrode 12 and the positive electrode 16. Binders andother materials normally associated with both the electrolyte and thenegative and positive electrodes are well known in the art and are notdescribed herein, but are included as is understood by those of ordinaryskill in this art. FIG. 10 shows a schematic illustration of one exampleof a battery in which two strings of electrochemical lithium cells 10,described above, are arranged in parallel, each string comprising threecells arranged in series.

[0046] While particular embodiments of the present invention have beenshown and described, it will be appreciated by those skilled in the artthat changes and modifications may be made without departing from thetrue spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A negative electrode fora non-aqueous electrochemical cell comprised of an intermetalliccompound having as its basic structural unit the formula M₂M′ in which Mand M′ are selected from two or more metal elements including Si, andthe M₂M′ structure is a Cu₂Sb-type structure.
 2. A negative electrode ofclaim 1, in which M₂M′ has a fully disordered structure or a partiallydisordered structure.
 3. A negative electrode of claim 1, in which M₂M′is a non-stoichiometric compound in which the M:M′ ratio falls withinthe range 2.33:1 to 1.67:1.
 4. A negative electrode of claim 3, in whichM₂M′ is a non-stoichiometric compound in which the M:M′ ratio fallswithin the range 2.1:1 to 1.9:1.
 5. A negative electrode of claim 1, inwhich M is Cu, Mn and/or Li, and M′ is Sb.
 6. A negative electrode ofclaim 5, in which the Li content if present is 20 atom percent or lessof the M₂M′ structure.
 7. A negative electrode of claim 1, in which M₂M′is Cu₂Sb or Mn₂Sb.
 8. A negative electrode of claim 1, in which Li isadded as a separate component to the M₂M′ electrode to the extent of 20atom percent or less of the M₂M′ structure.
 9. A negative electrode ofclaim 1, in which surplus M metal is added to the M₂M′ electrode, to theextent of 50 atom percent or less.
 10. A negative electrode of claim 9,in which the surplus M metal is 20 atom percent or less.
 11. A negativeelectrode for a non-aqueous electrochemical cell comprised of a compoundhaving as its basic structural unit the formula M₂M′, and having aCu₂Sb-type structure, in which M is one or more metal elements and M′ isone or more metal or non-metal elements.
 12. A negative electrode ofclaim 11, in which the M′ atoms are selected from the Group Va elementsexcluding nitrogen.
 13. A negative electrode of claim 12, in which theM′ atoms are selected from P or As.
 14. A non-aqueous electrochemicalcell comprising a negative electrode, an electrolyte and a positiveelectrode the negative electrode including an intermetallic compoundhaving as its basic structural unit the formula M₂M′ in which M and M′are selected from two or more metal elements including Si, and the M₂M′structure is a Cu₂Sb-type structure.
 15. A battery comprising of aplurality of cells, at least some cells including a negative electrodeand a non-aqueous electrolyte and a positive electrode, said negativeelectrode having an intermetallic compound having as its basicstructural unit the formula M₂M′ in which M and M′ are selected from twoor more metal elements including Si, and having a Cu₂Sb-type structure.16. A non-aqueous electrochemical cell comprising a negative electrode,an electrolyte and a positive electrode, said negative electrode havingan intermetallic compound having as its basic structural unit theformula M₂M′, and having a Cu₂Sb-type structure, in which M is one ormore metal elements and M′ is one or more metal or non-metal elements.17. A battery comprising a plurality of cells, at least some cellsincluding a negative electrode and a non-aqueous electrolyte and apositive electrode, said negative electrode including an intermetalliccompound having as its basic structural unit the formula M₂M′, and theM₂M′ structure is a Cu₂Sb-type structure, in which M is one or moremetal elements and M′ is one or more metal or non-metal elements.